Many studies have been made to increase the recording density of magnetic recording media (i.e., [the relative speed of tape and head]/[maximum frequency of recording signals]) so as to make high-fidelity audio cassettes and low-speed small video tapes.
The proposals made to date include increasing the coercive force of the magnetic layer, increasing both the residual magnetization and coercive force (as in a metal tape), and increasing the coercive force of an upper magnetic layer.
Vertical magnetic recording has also been proposed wherein a residual magnetizable component in a direction normal to the plane of the magnetic recording medium is effectively utilized. According to this method, the recording density as defined above is increased, and when the recording wavelength is made smaller than the thickness of the recording layer, there is no further reduction in output due to self-demagnetization ("self-demagnetization" means the decrease in residual magnetization due a self-demagnetizing field that is established between the N-S poles, created by its own residual magnetization, and that exists in a direction opposite to that of its magnetization so as to offset the latter, and the magnitude of self-demagnetization is increased with decreasing distance between the N-S poles). Thus, this method is said to be effective in high-density recording in a region where the recording wavelength is less than 1 or 2 microns.
Another proposal is to utilize a magnetic layer of a coated type containing a magnetizable component extending obliquely or normally to the magnetic surface. See U.S. Pat. Nos. 3,052,567 and 3,185,775, and Japanese Patent Publication No. 15203/74.